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<h2>Exception Asserts (NUnit 2.5)</h2>

<p>The <b>Assert.Throws</b> method is pretty much in a class by itself. Rather than
   comparing values, it attempts to invoke a code snippet, represented as
   a delegate, in order to verify that it throws a particular exception.
   
<p>It's also in a class by itself in that it returns an Exception, rather
    than void, if the Assert is successful. See the example below for
	a few ways to use this.
   
<p><b>Assert.Throws</b> may be used with a constraint argument, which is applied
   to the actual exception thrown, or with the Type of exception expected.
   The Type format is available in both both a non-generic and (in the .NET 2.0 version)
   generic form.
   
<p><b>Assert.DoesNotThrow</b> simply verifies that the delegate does not throw
   an exception.
   
<p><b>Assert.Catch</b> is similar to <b>Assert.Throws</b> but will pass for an exception
   that is derived from the one specified.
   
<div class="code" style="width: 40em"><pre>
Exception Assert.Throws( Type expectedExceptionType, TestDelegate code );
Exception Assert.Throws( Type expectedExceptionType, TestDelegate code, 
		string message );
Exception Assert.Throws( Type expectedExceptionType, TestDelegate code, 
		string message, params object[] parms);

Exception Assert.Throws( IResolveConstraint constraint, TestDelegate code );
Exception Assert.Throws( IResolveConstraint constraint, TestDelegate code, 
		string message );
Exception Assert.Throws( IResolveConstraint constraint, TestDelegate code, 
		string message, params object[] parms);

T Assert.Throws&lt;T&gt;( TestDelegate code );
T Assert.Throws&lt;T&gt;( TestDelegate code, string message );
T Assert.Throws&lt;T&gt;( TestDelegate code, string message, 
		params object[] parms);
		
void Assert.DoesNotThrow( TestDelegate code );
void Assert.DoesNotThrow( TestDelegate code, string message );
void Assert.DoesNotThrow( TestDelegate code, string message, 
        params object[] parms);

Exception Assert.Catch( TestDelegate code );
Exception Assert.Catch( TestDelegate code, string message );
Exception Assert.Catch( TestDelegate code, string message, 
        params object[] parms);

Exception Assert.Catch( Type expectedExceptionType, TestDelegate code );
Exception Assert.Catch( Type expectedExceptionType, TestDelegate code, 
		string message );
Exception Assert.Catch( Type expectedExceptionType, TestDelegate code, 
		string message, params object[] parms);

T Assert.Catch&lt;T&gt;( TestDelegate code );
T Assert.Catch&lt;T&gt;( TestDelegate code, string message );
T Assert.Catch&lt;T&gt;( TestDelegate code, string message, 
		params object[] parms);
</pre></div>

<p>In the above code <b>TestDelegate</b> is a delegate of the form
<b>void TestDelegate()</b>, which is used to execute the code
in question. Under .NET 2.0, this may be an anonymous delegate.
If compiling under C# 3.0, it may be a lambda expression.

<p>The following example shows different ways of writing the
same test.

<div class="code"><pre>
[TestFixture]
public class AssertThrowsTests
{
  [Test]
  public void Tests()
  {
    // .NET 1.x
    Assert.Throws( typeof(ArgumentException),
      new TestDelegate(MethodThatThrows) );
	  
    // .NET 2.0
    Assert.Throws&lt;ArgumentException&gt;( MethodThatThrows() );

    Assert.Throws&lt;ArgumentException&gt;(
	  delegate { throw new ArgumentException(); } );

    // Using C# 3.0	  
    Assert.Throws&lt;ArgumentException&gt;(
      () => throw new ArgumentException(); } );
  }
  
  void MethodThatThrows()
  {
    throw new ArgumentException();
  }

</pre></div>

<p>This example shows use of the return value to perform
additional verification of the exception.

<div class="code"><pre>
[TestFixture]
public class UsingReturnValue
{
  [Test]
  public void TestException()
  {
    MyException ex = Assert.Throws&lt;MyException&gt;(
      delegate { throw new MyException( "message", 42 ); } );
    Assert.That( ex.Message, Is.EqualTo( "message" ) );
    Assert.That( ex.MyParam, Is.EqualTo( 42 ) ); 
  }
</pre></div>

<p>This example does the same thing
using the overload that includes a constraint.

<div class="code"><pre>
[TestFixture]
public class UsingConstraint
{
  [Test]
  public void TestException()
  {
    Assert.Throws( Is.Typeof&lt;MyException&gt;()
                 .And.Message.EqualTo( "message" )
                 .And.Property( "MyParam" ).EqualTo( 42 ),
      delegate { throw new MyException( "message", 42 ); } );
  }
</pre></div>

<p>Use the form that matches your style of coding.

<h3>Exact Versus Derived Types</h3>

<p>When used with a Type argument, <b>Assert.Throws</b> requires
that exact type to be thrown. If you want to test for any
derived Type, use one of the forms that allows specifying
a constraint. Alternatively, you may use <b>Assert.Catch</b>,
which differs from <b>Assert.Throws</b> in allowing derived
types. See the following code for examples:

<div class="code">
<pre>
// Require an ApplicationException - derived types fail!
Assert.Throws( typeof(ApplicationException), code );
Assert.Throws&lt;ApplicationException&gt;()( code );

// Allow both ApplicationException and any derived type
Assert.Throws( Is.InstanceOf( typeof(ApplicationException), code );
Assert.Throws( Is.InstanceOf&lt;ApplicationException&gt;(), code );

// Allow both ApplicationException and any derived type
Assert.Catch&lt;ApplicationException&gt;( code );

// Allow any kind of exception
Assert.Catch( code );
</pre>
</div>

<h4>See also...</h4>
<ul>
<li><a href="throwsConstraint.html">ThrowsConstraint</a></ul>

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